The use of an electric motor for adjusting a plate cylinder of an offset printing press is well known. Typically, a plate cylinder of a printing press is circumferentially adjusted, laterally adjusted, and/or phase adjusted. In a multi-unit printing press, a motor is associated with each plate cylinder to adjust the circumferential position of the plate cylinder, for example. The motors are controlled to adjust their respective plate cylinders at a constant rate.
Poor quality or doubling of a printed image on a web of sheet material may occur because of an adjustment of the plate cylinder by the prior known technique. Poor quality or doubling occurs because the amount of adjustment per unit of time, i.e. the rate of adjustment, of the plate cylinder is constant. Since the rate of adjustment of the plate cylinder is constant, the amount of adjustment per revolution of the plate cylinder is not constant but varies as a function of the rate of revolution, i.e. the rotational speed, of the plate cylinder.
For example, when a plate cylinder and a cooperating blanket cylinder in an offset printing press are rotating together at a fast rate, the amount of adjustment per revolution of the plate cylinder is relatively small. However, when the plate cylinder and the blanket cylinder are rotating together at a slow rate, the amount of adjustment per revolution of the plate cylinder is relatively large. When the amount of adjustment per revolution of the plate cylinder is relatively large, the plate cylinder could be advanced to an extent such that doubling occurs.
Doubling occurs because the blanket cylinder prints the same image onto a web of sheet material twice. One printing is due to the residual (leftover) ink of an image which is applied by the printing plate on the plate cylinder to one location on the blanket cylinder during one revolution of the blanket cylinder and remains on the blanket cylinder after the one revolution of the blanket cylinder. The other printing is due to ink of the same image which is applied by the printing plate on the plate cylinder to another location on the blanket cylinder after the one revolution of the blanket cylinder and after adjustment of the plate cylinder. The image on the web from the one printing and the image on the web from the other printing are offset from each other enough give the appearance of a double image, i.e. doubling of an image. When doubling occurs, poor quality and/or wasted paper results.